Public cellular networks (public land mobile networks) are commonly employed to provide voice and data communications to a plurality of subscribers. For example, analog cellular radiotelephone systems, such as designated AMPS, ETACS, NMT-450, and NMT-900, have been deployed successfully throughout the world. More recently, digital cellular radiotelephone systems such as that designated as IS-54B (and its successor IS-136) in North America and the pan-European GSM system have been introduced. These systems, and others, are described, for example, in the book titled Cellular Radio Systems by Balston, et al., published by Artech House, Norwood, Mass., 1993. In addition, satellite based radio communication systems are also being utilized to provide wireless communications in various regions such as the Asian Cellular Satellite System (ACeS) generated by Lockheed Martin Corporation. Furthermore, dual-mode mobile terminals are known which allow a single terminal to access to different networks. For example, an analog/digital dual-mode terminal or a terrestrial/satellite dual-mode terminal may be desirable in various geographic areas to maximize the communications capabilities available to a user.
FIG. 1 illustrates a conventional terrestrial wireless communication system 20 that may implement one of the aforementioned wireless communication standards. The wireless system may include one or more wireless mobile terminals 22 that communicate with a plurality of cells 24 served by base stations 26 and a mobile telephone switching office (MTSO) 28. Although only three cells 24 are shown in FIG. 1, a typical cellular radiotelephone network may comprise hundreds of cells, and may include more than one MTSO 28 and may serve thousands of wireless mobile terminals 22.
The cells 24 generally serve as nodes in the communication system 20, from which links are established between wireless mobile terminals 22 and a MTSO 28, by way of the base stations 26 servicing the cells 24. Each cell 24 will have allocated to it one or more dedicated control channels and one or more traffic channels. The control channel is a dedicated channel used for transmitting cell identification and paging information. The traffic channels carry the voice and data information. Through the communication system 20, a duplex radio communication link 30 may be effected between two wireless mobile terminals 22 or between a wireless mobile terminal 22 and a landline telephone user 32 via a public switched telephone network (PSTN) 34. The function of the base station 26 is commonly to handle the radio communications between the cell 24 and the wireless mobile terminal 22. In this capacity, the base station 26 functions chiefly as a relay station for data and voice signals.
FIG. 2 illustrates a conventional celestial wireless communication system 40. The celestial wireless communication system 40 may be employed to perform similar functions to those performed by the conventional terrestrial wireless communication system 20 of FIG. 1. In particular, the celestial wireless communication system 40 typically includes one or more satellites 42 that serve as relays or transponders between one or more earth stations 44 and satellite wireless mobile terminals 23. The satellite 42 communicates with the satellite wireless mobile terminals 23 and earth stations 44 via duplex communication links 46. Each earth station 44 may in turn be connected to a PSTN 34, allowing communications between the wireless mobile terminals 23 and conventional landline telephones 32 (FIG. 1).
The celestial wireless communication system 40 may utilize a single antenna beam covering the entire area served by the system, or as shown in FIG. 2, the celestial wireless communication system 40 may be designed such that it produces multiple, minimally-overlapping beams 48, each serving a distinct geographical coverage area 50 within the system's service region. A satellite 42 and coverage area 50 serve a function similar to that of a base station 26 and cell 24, respectively, of the terrestrial wireless communication system 20.
Thus, the celestial wireless communication system 40 may be employed to perform similar functions to those performed by conventional terrestrial wireless communication systems. In particular, a celestial radiotelephone communication system 40 has particular application in areas where the population is sparsely distributed over a large geographic area or where rugged topography tends to make conventional landline telephone or terrestrial wireless infrastructure technically or economically impractical.
Control over access to these various communication networks is typically provided by some form of user (subscriber) identification. In the earliest analog systems, such as the AMPS system, a subscriber identity verification is typically not required and charging for services is generally based upon the assigned phone number of the analog mobile terminal. With the introduction of digital and satellite based systems, a greater range of communication service options are typically made available to a user (subscriber) of a communication network such as paging and short message service. Accordingly, these networks typically require a subscriber identity verification from a mobile terminal attempting to access the network at the level authorized for the user's subscriber identity. For example, in the GSM system, a Subscriber Identity Module (SIM) is typically provided to a mobile terminal and provides subscriber identity which the terminal provides to the communication network provider to obtain access to the network. The network may then verify that the provided identification is valid before allowing access to the mobile terminal.
Access without a valid subscriber identification may vary across communication networks. For example, GSM allows emergency calls while ACeS allows no access. For normal operations, GSM-based digital systems typically require that a SIM card with a valid subscriber identity is present in the terminal when the mobile terminal is switched on. This subscriber identity may be verified on network request during mobile terminal network accesses and approved or rejected as a result of this process. Any SIM present in the phone when a verification process fails is typically considered invalid until the mobile terminal is switched off or the SIM card is replaced. If no valid SIM card is available at power on, or if the SIM card is removed or invalidated, systems typically provide only limited user service such as emergency calling. Other systems do not even provide emergency call access without a valid SIM.
A problem with dual-mode mobile terminals occurs where a subscriber identification is lost. The assigned identification may be lost due to failure to pay or other act by the subscriber or due to communication errors during an exchange of identification information with a communication network. Where access to one network is affected, the mobile terminal's protocols for selecting an operating mode may be less than optimal for user performance.